[TRACKING] Implement Low-income eligibility and enrollment pathway

[thekaveman]

Based on design specs in #1677

Acceptance Criteria

• Session stores enrollment expiration, context processor makes available to templates #1981
• Include expiry_date with all CalFresh / Low income enrollments #1877
• Create new CalFresh Cardholder informational modal #1918
• Eligibility index: Add CalFresh Cardholder option #1915
• Eligibility start: Update “You selected an CalFresh Cardholder transit benefit.” #1916
• Enhance EligibilityType model with expiration-related field(s) #1938
• Enrollment index: Update for CalFresh #1917
• Enrollment success: Add expiration date text #1919
• Help page updates for CalFresh information #1920
• Create new Enrollment error state for users already enrolled, not expired yet / not within re-enrollment window #1921
• Translate new CalFresh strings #1941
• Bring back the expiry date logic #2062
• Refactor enrollment check to lookup groups for funding source #2068
• QA Spanish copy on test-benefits.calitp.org for CalFresh Cardholders #1971